When creating vendor record, when I select non-US subsidiary and enter a US address for vendor, why do I still see Vat Reg Num and not Tax ID, any idea?

Are you using a custom form that need to be changed?

Hmm - I'm but when I select a US subsidiary the Tax ID field is exposed so looks like something in the background is driving this change based on subsidiary country

Its inherent to NetSuite. same field. label updates to assist local users...